1. Field of the Invention
The present invention relates to a bus terminating circuit. More specifically, the present invention relates to a circuit for terminating a bus line between computer equipment which utilizes an SCSI interface (small computer system interface), for example, and a method for connecting or disconnecting terminating resistors to or from the bus line.
2. Description of the Prior Art
Conventionally, as standard interfaces for small sized computers, "GP-IB" which is mainly used in the field of automatic measurement, "RS-232C" which is mainly used in the field of communication, etc., were known. The interface is used by being incorporated in a host computer or by being added to a board as an option. The interface has electric or electronic specifications and an execution procedure both determined dependent on a required function, and the interface and the host computer are connected to each other through a bus line at portions where electrical levels become identical to each other.
An interface according to an SCSI standard also accesses an SCSI bus line via a host adapter. In an SCSI system, an apparatus such as the host computer sending a command is called an initiator and an apparatus such as a hard disk, printer or the like executing the command is called a target. However, in the SCSI system, according to circumstances, it is allowed that the initiator becomes a target and the target becomes an initiator inversely. The initiator and the target are connected to each other by the SCSI bus line. All the equipment existing on the SCSI bus line are called SCSI devices irrespective of whether they are initiators or targets. On the SCSI bus line, typically, eight SCSI devices at most can be connected.
Signals for the SCSI device are as shown in FIG. 1, for example. In FIG. 1, a reference numeral 1 denotes an initiator and a reference numeral 2 denotes a target. The SCSI bus line includes eighteen signal lines in total, composed of nine upper data signal lines (including one line for an odd parity bit) and nine lower control signal lines. On each of the signal lines, a binary signal "0" or "1" is transmitted.
One example of a connection between the initiator 1 and the target 2 is shown in FIG. 2. FIG. 2 shows a connection called a single ended type. As shown in FIG. 2, the initiator 1 and the target 2 are connected to each other through an SCSI bus line 3. A reference numeral 4a denotes a power source line to which a power source voltage Vcc (normally, 5 volts) is applied. A reference numeral 4b denotes a single signal line as shown in FIG. 1 and, although not shown, the SCSI bus line 3 actually includes eighteen(18) signal lines as described above. A reference numeral 4c denotes a ground line and, in fact, a plurality of ground lines are provided. The power source line 4a, signal lines 4b and ground lines 4c are put together and called the bus line 3.
A reference numeral 5 denotes a terminator which includes two terminating resisters 5a and 5b connected in series between the power source line 4a and the ground. The terminator 5 is provided for the purpose of impedance matching so that the binary signal can be transmitted as an accurate square wave on the SCSI bus line 3 between the initiator 1 and the target 2. Therefore, a series connection point A of the terminating resistor 5a and 5b is connected to the signal line 4b. Such a terminator 5 is provided in each SCSI device. In the SCSI system a number of terminators 5 corresponding to the number of signal lines are provided for each SCSI device. In addition, in a case of the single ended type shown as in FIG. 2, resistance values of the terminating resistor 5a and 5b are defined as 220 ohms and 330 ohms, respectively.
Next, one example of the SCSI system is shown in FIG. 3. In FIG. 3, reference numerals 2a, 2b, 2c and 2d respectively show the target 2 such as a printer, floppy disk, scanner, hard disk, etc. In a case where a plurality of SCSI devices are connected to each other as shown in FIG. 3, depending on the specification of the SCSI system, the terminating resistors of the devices (in the case of FIG. 3, the targets 2a, 2b and 2c) connected between the devices at both ends (in the case of FIG. 3, the initiator 1 and the target 2d) on the SCSI bus line 3 must be disconnected from the bus line 3. Conventionally, for connecting/disconnecting the terminating resistors, two methods were known, (i) a method where mechanical switches such DIP switches are provided at positions shown by the reference symbol A in FIG. 2 and are turned on or off, and (ii) a method where a connector or resistor module which incorporates the terminator 5 (in FIG. 2, the two terminating resistors 5a and 5b between positions denoted by reference symbols B and C) is attached or detached.
However, in the former method, a number of switch contacts corresponding to the number of the signal lines (eighteen in the case of FIG. 2) are needed. In addition, even when the switch (not shown) is turned-off, the terminating resistors 5a and 5b are always supplied with the power source voltage from the power source line 4a (FIG. 2), and therefore, an electric power of approximately 0.8 W is consumed in each device. Therefore, there was a disadvantage that electric power is wastefully consumed in the targets 2a, 2b and 2c shown in FIG. 3, just as power is consumed when the terminating resistors thereof are connected to the bus line.
In the latter method, unlike the former method, no electric power is consumed at the terminating resistors if the same are disconnected from the bus line; however, it requires maintenance to keep the connector incorporating the terminating resistors or the terminating resistor module. Therefore, conventionally, there was a problem that the above described connector or module may be missed. In addition, since the connector or module is manually attached to or detached from the device, in a recent situation where a plurality of devices must be connected in a complex arrangement, the work required for attachment or detachment of the connector or module is troublesome, and therefore, this troublesomeness becomes a large obstacle to changing the system.
Furthermore, in a case of the single ended type as shown in FIG. 2, the characteristic impedance of the signal line is generally less than 110 ohms, while the impedance of the terminator 5 viewed from the signal line 4b becomes 132 ohms. Therefore, a mismatch easily occurs, and therefore, to that extent, the bus line (cable) cannot be made longer and the transmission rate of a signal cannot be made greater.
Therefore, in order to solve such problems, the SCSI-2 system according to a revised version was proposed. One example of a bus line for the SCSI-2 system is shown in FIG. 4. In FIG. 4, a reference numeral 5c denotes a terminating resistor which has a resistance value of 110 ohms. An input of a 3-terminal regulator 6 is connected to the power source line 4a, and the regulator 6 adjusts the power source voltage 5 to a level of approximately 2.85 volts which is supplied to the terminating resistor 5c from an output thereof. Between the power source line 4a and the ground line 4c, and between the series connection point of the regulator 6 and the terminating resistor 5c and the ground line 4c, capacitors 7 are connected. These capacitors 7 are provided for purpose of elimination of a noise and ripple on the power source line 4a and the output line of the regulator 6.
In the circuit for terminating the bus line 3 shown in FIG. 4, the above-described problems still remain with respect to the connection/disconnection of the terminating resistor.
Furthermore, there was a similar problem in the SCSI bus line of a differential type as shown in FIG. 5. The SCSI bus line 3 shown in FIG. 5 includes eighteen(18) first signal lines 4b and eighteen(18) second signal lines 4b'. Each second signal line 4b' transmits a signal which is the of a signal transmitted on a corresponding one of the first signal lines 4b. There is further single power source line 4a and one or more ground lines 4c. More specifically, by a driver 4c' and a receiver 4d which are complementarily combined with the first signal line 4b and the second signal line 4b', a signal level on the first signal line 4b and a signal level on the second signal line 4b' are inverted with respect to each other. Further, a terminator 5 includes three terminating resistors 5a, 5c and 5b connected in series between the power source line 4a and ground, and a series connection point A of the terminating resistors 5c and 5b is connected to the first signal line 4b and a series connection point A' of the terminating resistors 5a and 5c is connected to the second signal line 4b'. In the case of the differential type as shown in FIG. 5, the terminating resistors 5a, 5b and 5c are defined as 330 ohms, 330 ohms and 150 ohms, respectively. Then, by comparing a voltage level of the first signal line 4b and a voltage level of the second signal line 4b' with each other, "1" or "0" can be determined.
In the terminating circuit for the SCSI bus line 3 of the differential type as shown in FIG. 5, if the aforementioned connection/disconnection method (i) is utilized, mechanical switches are provided at positions shown by reference symbols A and A'. Therefore, in the example shown in FIG. 5, fifty-four(54) switches are required.